You've got almost 3 months to get ready for this thing. If you've done a 50 and had energy to spare, then you can easily prepare for this with the time that you have. Stay in shape and continue to ride regularly - Pre-ride as much of the route as you can - the route may not have hills but there could be prevailing winds that will be helpful to know about, and it also gives you a feeling of home court advantage on ride day. Try to get in at least two 75 mile rides between now and then to help prepare you psychologically for the longer distance and added saddle time - As others have said, it's not just a physical jump going from a 50 mile ride to a 100, your first century is also a mental jump.

I can't help you re: the friend thing. I'm also an introvert and love being alone on the bike - To me, the best partners are the ones who shut up and ride. If you're in shape and mentally prepared for the ride then not having a partner shouldn't matter.